shouldn't we add `Add` for `Cow<'a, str>`?

[llogiq]

After some experience, I've changed my mind in Addition for strings. Let's do it for Cow too. The rationale is that we already have the addition via Deref and we can optimize for the case of adding empty strings (or Cows of them, respectively) – we can get rid of one clone in that case.

[llogiq]

This is now in stable, so it seems safe to close.